select t1.id, t1.qty-t2.qty as qty from tblProductStock as t1 inner join
(
select id, sum(qty) as qty from tblStockMinus group by id
) as t2 on t1.id=t2.id

You need to update this (but it depends in the RDBMS. This is for MS SQL Server)

Update t
set
t.qty=t3.qty from tblProductStock as t inner join
(
select t1.id, t1.qty-t2.qty as qty from tblProductStock as t1 inner join
(
select id, sum(qty) as qty from tblStockMinus group by id
) as t2 on t1.id=t2.id
) as t3 on t.id=t3.id